1. Technical Field
The present invention relates to a control apparatus for an alternating-current (AC) motor.
2. Related Art
From a societal need of low fuel consumption and low exhaust emission, electric cars and hybrid cars mounting an AC motor as a power source of the cars are now gaining attention. For example, in a hybrid car, a direct-current (DC) power source composed such as of a secondary battery is connected to an AC motor via a power converter which is configured such as by an inverter. A DC voltage outputted from the DC power source is converted to an AC voltage by the inverter to thereby drive the AC motor.
Such a hybrid car or an electric car is mounted with a control apparatus used for the AC motor. A patent document JP-A-2007-185043, for example, discloses that, in such a control apparatus, two current sensors are provided per phase and, when the difference between the two current sensor values exceeds a threshold, one of the current sensors is determined to have a fault. Further, in this control apparatus, waveforms of the current sensor values of a different phase with an offset of 120° in phase are generated. The so generated waveforms are compared with the current detection values of the phase, in which one of the current sensors has a fault, to specify a normal current sensor value, and the normal current sensor value is used for continuing feedback control.
Another patent document JP-A-2008-086139 discloses that three-phase alternating current commands are obtained by subjecting d- and q-axis current commands and an electrical angle to reverse d-q conversion. In addition to current detection values of one phase as a sensor phase, the three-phase alternating current commands are used as current estimate values for the phases other than the sensor phase.
In JP-A-2007-185043, when one of the current sensors provided in one phase has a fault, the current detection value of the other current sensor that is operating normally is used for the continuation of control. Accordingly, if one of the current sensors has a fault, the current sensor continuously used for control cannot be monitored and thus the monitoring performance is lowered.
In JP-A-2008-086139, control is constantly performed on the basis of the current detection values of one phase. Therefore, for example, in the event that an abnormal current is generated due to a fault or the like of a current sensor, it is difficult to properly capture the state of the fault for the control of the AC motor. Hereinafter, the control using the current detection values of one phase is referred to as one-phase control and the control using current detection values of two phases is referred to as two-phase control.